Impact of rod-dominant mesopic conditions on spatial summation and surround suppression in early visual cortex.

Mesopic (dim light) conditions are prevalent in everyday environments, yet most human vision research is conducted under idealized, photopic (bright) conditions. Electrophysiological studies suggest that under mesopic conditions, retinal ganglion cell receptive fields, which encode contrast, expand their center width while diminishing surround inhibition. These retinal modifications enhance light capture by increasing the summation area but they limit spatial resolution. However, the impact of mesopic conditions on human cortical spatial integration mechanisms remains unclear. To address this, we investigate how mesopic conditions affect early visuocortical processing, specifically spatial summation and surround suppression. Across two experiments, we acquired fMRI BOLD responses from 11 normally-sighted participants of both sexes under photopic and mesopic conditions in visual areas V1 - V3. The first experiment estimated population receptive field (pRF) properties while the second experiment assessed cortical surround suppression. Photopic and mesopic psychophysical surround suppression, Contrast Sensitivity Function (CSF), and visual acuity were also measured. At the cortical level, mesopic conditions were associated with smaller pRF sizes, while surround suppression remained robust. At the perceptual level, mesopic conditions led to reduced acuity, lower CSF, and weaker suppression, diverging from the observed cortical effects. Importantly, individual differences linked these findings: participants who exhibited greater mesopic reductions in visual acuity also showed larger decreases in early visuocortical surround suppression, underscoring its role in contrast coding and spatial resolution. Altogether, our fMRI findings contrast with retinal electrophysiology and suggest that early visual cortex may employ distinct, perhaps compensatory, mechanisms in response to reduced retinal input under mesopic conditions.Significance Statement Despite the prevalence of mesopic (dim light) environments, their impact on human visuocortical processing remains understudied. Electrophysiological studies suggest that mesopic conditions lead to larger receptive fields and reduced surround inhibition in retinal ganglion cells, enhancing light summation at the cost of spatial resolution. Using fMRI and psychophysical measurements, we investigate how mesopic conditions impact spatial summation and surround suppression across early visual cortex. We find that under mesopic conditions, population receptive fields become smaller, and cortical surround suppression remains robust. However, individual differences revealed a correlation between mesopic visual acuity impairment and changes in V1 surround suppression. These findings contrast with retinal electrophysiological findings, pointing to potential cortical refinement mechanisms that help preserve visual function under degraded viewing conditions.